MBP A1297 - pulsing PP5V_S3/PP5V_S0

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  • cruiser_
    Senior Member
    • Nov 2019
    • 124
    • Ireland

    #1

    MBP A1297 - pulsing PP5V_S3/PP5V_S0

    hi folks, scratching my head with this one... please help

    MB: 820-2914-A

    history: just turned off, no watermarks

    initial issue: short on PPBUS_G3H - gone by removing elements in the GPU power rail path
    current issue: pulsing most of S3+ rails ( and PM_SLP* ) - every few seconds. Pulse lenght about half a second.

    found/fixed so far:
    - shorted GPU (now removed all the power to it - coils, U8900, some FETs)
    - U7201's P3V3S5_PGOOD (S5_PWRGD) - all time ok
    - U7201's P5VS3_PGOOD - on for a very short spike (ms)
    - no shorts on other coils (especially CPU ones)

    going PP5V_S3 path (which produces PP5V_S0) all checked out with exception of U7100, which exhibits behavior similar to that of U7201:
    - PVCCSA_PGOOD - very short spike (ms)
    - FB pins 7/11/12 - longer pulse (hundreds of ms)
    - pin 5 (CPU_VCCSA_VID) 1.05V

    So to sum up - the enable signals are there for longer period (half a second), which translates to very short (ms) PGOOD signals for PP5V_S3 / PP5V_S0, which translates into a power loop.

    No short on PPVCCSA_S0_CPU. Regular 1.05V pulsing as above (half a second output).

    Thing which is off about U7100 is that there is a voltage divider between pins 7,8 & 9. I am not getting anything on the 'bottom' of R7148 (pin 2), even though the resistor itself checks out as 140K and has 0.51V on pin 1. Not sure if this is at all related...

    What gives?
    Attached Files
    Last edited by cruiser_; 12-26-2022, 07:26 AM.
  • dellxps15
    Badcaps Legend
    • Feb 2014
    • 1588
    • italy

    #2
    Re: MBP A1297 - pulsing PP5V_S3/PP5V_S0

    check bios voltage.
    check cpu area for corrosion or similar.

    Comment

    • cruiser_
      Senior Member
      • Nov 2019
      • 124
      • Ireland

      #3
      Re: MBP A1297 - pulsing PP5V_S3/PP5V_S0

      spi chip voltage is there (PP3V3_S5 is stable). Some activity on CS line too, single spike on CLK and little on MISO/MOSI. Board is pretty dirty and all but I haven't spotted any corrosion in the area (nor anywhere else but empty pads).

      doh... this one was a tricky one...got it to POST.

      I didn't 'cut off' all the power lines to the GPU, there was one still shorted in output of U7880, which was pulling too much current from PP1V8_S0, resulting in P1V8S0_PGOOD going down from U7720.

      This would bring ALL_SYS_PWRGD down. In this case ALL_SYS_PWRGD is tied directly to some of the PGOOD signals from above, so it was actually ALL_SYS_PWRGD bringing them down, rather than them bringing ALL_SYS_PWRGD down

      The board now chimes & boots, but there is no backlight nor anything on screen.

      Anyone familiar with hardware GPU disable stuff why would this be the case? Thought I'd get the internal card's output & working backlight during boot.
      Last edited by cruiser_; 12-27-2022, 07:00 AM.

      Comment

      • cruiser_
        Senior Member
        • Nov 2019
        • 124
        • Ireland

        #4
        Re: MBP A1297 - pulsing PP5V_S3/PP5V_S0

        opened a new thread now on backlight issue...
        https://www.badcaps.net/forum/showthread.php?t=113813

        Comment

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